Arrangement for the Determination of the Effective Pulmonary Blood Flow
The invention relates to an arrangement for the determination of the effective pulmonary blood flow according to the generic part of claim 1.
From the paper Steinhart, C. M., Burch, K. D., Bruno, S., Parker, D. H.: Noninvasive determination of effective (nonshunted) pulmonary blood flow in normal and injured lungs, Crit. Care Med., 1989, Vol. 17, No. 4, pp. 349-353 the Multiple-Inert-Gas method with rebreathing of helium, acetylene and carbon monoxide in oxygen and nitrogen from a respiratory bag is known. A disadvantage of this measurement is the relatively great effort to nitrogen from a respiratory bag is known. A disadvantage of this measurement is the relatively great effort to prepare the gas mixtures for rebreathing, the demand for special measuring instruments that can measure the concentrations of gases in the breathed air or the breathing flow, respectively, and the necessity of a person to connect the respiratory bag to the patient and maintain it. For those reasons the measurement of the effective pulmonary blood flow can only be executed by specialists and is for research purposes in almost all cases.
Further, from the paper by Inman, M. D., Hughson, R. L., and Jones, N. L.: Comparison of cardiac output during exercise by single-breath and CO2 rebreathing methods, J. Appl. Physiol., Vol. 58, pp. 1372-1377, 1985, the total CO2 rebreathing method and the so-called single-breath method are known. Disadvantages of these methods are distinct increases of CO2 pressure in the arterial blood, as the CO2 elimination is interrupted, and the impeding of the respiration. Other disadvantages are those of the Multiple-Inert-Gas method, namely the preparation of gas mixtures for rebreathing having certain CO2 concentrations, the demand for special measuring instruments that can measure the concentrations of gases in the breathed air or the breathing flow, respectively, and the necessity of a person to connect the respiratory bag to the patient and maintain it.
Another method known is a partial CO2 rebreathing method (according to Gedeon, A., Forslund, L., Hedenstierna, G. and Romano, E.: A new method for noninvasive beside determination of pulmonary blood flow, Med. and Biol. Eng. and Comp., 1980, Vol. 18, pp. 411-418) based on varying minute ventilation. Disadvantage of this method is the variation of the mean respiratory tract pressure and of the pressure at the end of expiration. Due to this variation of the respiratory tract pressure the pulmonary blood flow also varies and both the medical stability of the lungs and the gas exchange are impeded.
From the paper by Capek, J. M. and Roy, R. J.: Noninvasive measurement of cardiac output using partial CO2 rebreathing; IEEE Transactions on Biomedical Engineering, 1988, Vol. 35, No. 9, pp. 653-661) the partial CO2 rebreathing method with change of the dead space of the apparatus is known, which is performed using a mass spectrometer and special respiratory flow sensors at the endotracheal tube and can measure the total cardiac output. The change between two different dead spaces is performed by a PC-controlled electromagnetic valve. The partial expiration termination pressure and the CO2 elimination are determined for the respiration by both dead spaces. The CO2 partial pressure at the end of expiration is converted to the arterial CO2 concentration and the total cardiac output can be calculated from the division of the CO2 elimination difference by the arterial CO2 concentration difference. A disadvantage of this method is the demand for apparatus for the measurement of the CO2 concentration in the breathed air (mass spectrometer) and of the respiratory flow at the endotracheal tube (Fleisch pneumatocograph).
It is the objective of this invention to describe a clinically practicable arrangement for noninvasive determination of the effective pulmonary blood flow whereby the effective pulmonary blood flow is the cardiac output minus the intrapulmonary shunt proportion. Only that portion of the cardiac output is intended to be determined which is available for gas exchange. This arrangement requires only little apparatus, does not essentially influence the respiratory schedule, and is capable of being automated.
According to the invention, the problem is solved using features given by claim 1. The dependant subclaims give other useful developments and embodiments.
The selected parameters, above all, made it possible for the first time to determine the effective pulmonary blood flow during respiration, i.e. only that portion of the cardiac output that is available for gas exchange.
The arrangement according to the invention, which is controlled by a microprocessor or a controller, the signals of a main stream CO2 sensor and a respiratory flow sensor are detected. The main stream CO2 sensor is intended for measurement of the CO2 concentration of the respiratory air, the respiratory flow sensor for measurement of the respiratory flow (FIG. 1). The respiratory flow sensor is located between the endotracheal tube and the CO2 sensor. There is a controllable 3-way valve between the endotracheal tube and the Y-fitting of the respirator. This valve is switched by the microprocessor or the controller so that the patient is respirated through a short or a long branch (so-called dead space) (FIG. 2).
Measurements of the CO2 elimination and expiration termination CO2 partial pressure are first performed during respiration through the small dead space. This period lasts approx. 60 s, and is called non-rebreathing period. After this period during an inspiratory cycle the 3-way valve is switched so that the patient is respirated through the bigger dead space (long branch) and rebreathes a gas mixture that consists of his or her own expired air and fresh air from the respirator. Thus no separate CO2 source for rebreathing is required. The time for switching the 3-way valve is derived from the absence of CO2 in the inspiratory air. This causes no essential variation of the respiratory pressure. The subsequent period last approx. 30 s is called rebreathing period. The CO2 elimination and the expiration termination CO2 partial pressure of this period are measured as mean values of each variable during a plateau that forms in the range of 15 to 30 s during this period (second half).
The arrangement according to the invention creates the possibility to set the respiratory schedule of the patient in the respirator such that the maximum pulmonary blood flow is achieved with the lowest mean and expiration termination respiratory tract pressure. This lowers the risk of the patient to suffer from a barotrauma, i.e. lung damage due to increased airway pressure is avoided and, simultaneously, the oxygen supply to the organs is optimised. This solution also raises the possibility to monitor the haemodynamics of the patient noninvasively and to record it automatedly. If simultaneous measurements of the cardiac output are taken, the found solution makes it possible to measure the percentage of the non-breathed cardiac output (so-called intrapulmonary shunt) without the inspiratory oxygen concentration being increased and blood samples being required.